The suspect in a shooting at a leading French newspaper office had lived in Britain for several years before returning to France in July, according to the Associated Press.
Abdelhakim Dekhar remains in custody in a suburban Paris hospital after police detained him on Wednesday night, the Paris prosecutor said.
Prosecutor Francois Molins told reporters that the witness who turned Dekhar in said he had been staying with him and that they had met while working in a restaurant together in London 13 years ago.
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Michael Fallon has received reassurances from his US counterpart that security ties with Britain will "endure" following a vote for Brexit.
On Friday, police said it appeared the girl had shot herself in the face with the weapon.
The showers will be slow-moving, heavy and thundery at times, particularly in central and eastern areas.